The present invention relates to the treatment of liquids, and specifically to a porous diffuser for the aeration and mixing of liquids.
The following words should be given the meanings below when interpreting the specification and claims. xe2x80x9cAeratexe2x80x9d shall mean to treat a liquid with air, oxygen or other gasses. xe2x80x9cAeration systemxe2x80x9d shall mean the system comprising all the components necessary to aerate a body of liquid. xe2x80x9cDiffuserxe2x80x9d shall mean an apparatus for aerating water or other liquids. xe2x80x9cGasxe2x80x9d shall mean any substance that is composed of constituent molecules that have the ability to expand indefinitely, as opposed to a solid or liquid. xe2x80x9cLiquidxe2x80x9d shall mean any substance that is composed of constituent molecules that move freely among themselves, but do not tend to separate like the molecules of gasses. xe2x80x9cPorous diffuserxe2x80x9d shall mean a diffuser composed of porous material, whether the diffuser is in the form of a hose, tube, sheet or other configuration. xe2x80x9cPurging mechanismxe2x80x9d shall mean any element (including, but not limited to, a stopper, cap, valve, piece of cloth etc.) inserted into or attached (permanently or removably) to the porous diffuser or the outlet pipe attached to the porous diffuser that can alternate between a closed position (defined as a position that prevents gas from exiting through the end of the porous diffuser or outlet pipe) and an open position (defined as a position that allows gas to exit through the end of the porous diffuser or outlet pipe).
In many instances, it is desirable to aerate liquids. For instance, in wastewater treatment plants, the bacteria that break down impurities in the wastewater during the treatment process depend on diffused oxygen in the water to flourish and complete the purification process. In aquaculture applications, such as the farming of fish, it is highly beneficial to maintain a stable, optimal concentration of diffused oxygen in the water to allow the fish to mature at optimal rates. Likewise, the ecology of lakes and streams depends on maintenance of a minimal concentration of oxygen to support the propagation of fish and other biota. As a result of pollution, the oxygen concentration in lakes and streams may decrease, with a corresponding decrease in the population of aquatic life. These are just several of the situations where aeration of liquids is appropriate, and is not meant to be an exhaustive list.
To address these situations, aeration systems have been employed. Several parameters impact the efficiency, operation and reliability of the aeration systems. Most aeration systems include some type of diffuser that releases gas into the liquid, usually in the form of bubbles, through holes or slits in the diffuser. Diffusers that produce a fine bubble pattern greatly increase the rate of gas absorption into the liquid as compared with diffusers that produce a coarse (larger) bubble pattern. As a direct result of the increased absorption rate, less energy is required to aerate a given body of liquid, which greatly decreases the cost of operation of the system (sometimes as much as a 50% reduction in energy costs). In addition, it is desirable to locate the diffusers of the aeration system in a total basin coverage pattern at or near the bottom of the body of liquid. This is so because bubbles produced by diffusers in such patterns reduce air flow rates, reduce the chimney effect caused by the upward velocity of the bubbles, and thus take longer to reach the surface. Hence, the bubbles have a longer time to transfer their gas to the surrounding liquid (referred to in the art as increased xe2x80x9cresidence timexe2x80x9d). Unlike some diffusers located on the side of a basin, which depend on upward velocities to aerate and mix the entire basin, total basin coverage arrangements can be used to aerate the basin as well as mix the basin due to their pattern of placement.
However, aeration systems with diffusers mounted over the entire bottom of the basin as described above, although more efficient in gas transfer, are more difficult to service, repair or replace than aeration systems with diffusers mounted on the side of a basin. Service, repair or replacement of the diffusers is a critical factor as the liquids to be treated often contain impurities (including, but not limited to, microscopic and macroscopic particulate matter, plant matter and living organisms). The impurities in the liquid become deposited in the diffuser as the water carrying the impurities seeps back into the diffuser during periods of non-use (when the gas is not being forced through the diffuser). This means the aeration system is prone to malfunction as the holes, slits or porous openings in the diffusers designed to release gas to the surrounding liquid become clogged or blocked. As a result, the operation of the aeration systems becomes suboptimal and service, repair or replacement of the diffusers is required to restore the aeration system to optimal operating conditions. As most of the aeration systems employ diffusers that are permanently fixed to the bottom of the basin, to access these diffusers requires that the basin be emptied of liquid (which in the case of a natural body of liquid is often impossible) or that dangerous activity on the surface of the liquid be undertaken. Finally, since large volumes of liquid are generally required to be treated, the aeration systems need to be economical to purchase, install and replace.
As an example of these difficulties, consider aeration systems designed for use in wastewater treatment plants. These aeration systems can include a series of supply pipes that supply pressurized gas, usually air or oxygen, to diffusers of various types. The diffusers normally take the form of numerous tubes, plates, domes or discs. These diffusers release gas into the liquid, generally as fine bubbles. The bubbles then transfer the gas into the surrounding liquid. Many of the aeration systems currently in commercial operation use porous diffusers made of ceramics, plastics or flexible sheaths to release gas into the liquid, while some diffusers use pipes with holes of slits to achieve this purpose. These porous materials, holes or slits eventually become blocked as liquid seeps into the diffuser through the porous material, holes or slits and deposits sediments or chemical precipitates into the diffuser, clogging them. As a result, the efficiency of the diffuser is greatly decreased and the diffuser must be serviced, replaced or repaired. In place service of a diffuser of the type described may consist of flushing the diffuser with hydrochloric acid, an operation which entails significant danger. If this hydrochloric acid treatment fails, then the diffusers must be repaired or replaced. Since the diffusers are generally permanently mounted at the bottom of the basin, repair or replacement of the diffusers involves draining the liquid from the basin to access the diffusers. This can be a costly and time consuming process. Of particular concern in the field of wastewater treatment, if the facility does not have two operational wastewater treatment basins, wastewater treatment operations must be suspended while the repair or replacement takes place. Additionally, when draining the basin it is frequently necessary to release raw sewage into the natural environment.
Therefore, a need exists for an aeration system, particularly for wastewater treatment applications, that uses a limited number of long, porous diffusers which can be located on the bottom of a basin in a full-floor pattern design which operates efficiently, is economical to install, is capable of extended operation without maintenance, and is easy to remove, repair or replace when necessary.